He even talked to the goron that tells him to do that! Arin looks at the fire in Darunia's room, looks at the bombs, looks at the unlit torch, then takes the long way back to the forest. I am laughing, but I am also indignant.

Yeah, the medical items in Isaac always intrigued me. I was thinking of suggesting an item idea involving Pica as an activated item that lets you eat bombs/keys/coins, but I never really developed the item past that.

You can get most of the unlocks on Normal, and the game really opens up after you start getting the more interesting unlockables, so it might be worth it to knock the difficulty down for now. It may also be worth it to do some of the easier challenges, or the ones that give good rewards, like Jera/Perthro. I'm on mobile, though, so you'd have to look the challenge rewards up yourself.

Challenges are typically one of two things: either teach you the mechanics of an item or be very hard. Usually, they're one item away from being broken!
As for whether Rebirth is worth it, I'd say it absolutely is. There is so much content in Rebirth that isn't in WOTL, and the game's better engine alone makes it worth it.